This invention relates to stabilization of satellites, particularly communication satellites, having antenna reflectors disposed symmetrically about a body of the satellite for directing beams of radiation towards sites on the surface of the earth and, more particularly, to the symmetrical mounting of plates of material which may be absorptive and/or reflective of radiant energy mounted on the back surfaces of the antenna reflectors to reduce torques upon the satellite produced by solar radiation pressure, thereby to stabilize the satellite against rotation resulting from such torque.
Satellites are placed in orbit around the earth for a variety of purposes, including communication, determination of positions of vehicles traveling near and upon the earth's surface, and measuring geophysical phenomena including the photographing of cloud cover for weather forecasting. Generally, such satellites include a pair of relatively large antenna reflectors which are angled symmetrically about a central plane of symmetry extending through a body of the satellite, each reflector having a generally elliptical shape which, upon projection in the direction of the earth, appears as a circular radiator due to the angle of inclination of the reflector to the plane of symmetry.
Optimum performance of the satellite mission is obtained by maintaining the satellite with a predetermined orientation relative to the surface of the earth and the earth's axis. Therefore, it is common practice to provide a satellite with some means for detecting a deviation from the desired orientation, as well as a means for reorienting the satellite to correct for such deviation. Such systems for the maintenance of a desired satellite orientation may be referred to generally as a satellite stabilization system. The accuracy with which a stabilization system can maintain satellite orientation is dependent, in part, upon the magnitude of disturbing influences such as satellite rotation inducing torques introduced by solar pressure on antenna elements. By way of analogy with a ship tossed on stormy waves at sea, greater stability can be maintained in calm waters than in stormy waters. Similarly, in the case of satellites, a reduction in the magnitude of external solar forces which may disturb a satellite orientation result in a more accurate maintenance of a desired orientation by a satellite stabilization system.
A problem arises in that solar radiation pressure, developed upon surfaces of the satellite by radiation incident thereon, is a primary factor in introducing torques which tend to rotate a satellite resulting in a deviation of a satellite's orientation from a desired orientation. The severity of the problem is exacerbated by the inclined orientation of the antenna reflectors relative to the plane of symmetry. Due to the angulation of the reflectors to the plane of symmetry, incident solar radiation impinges upon each of the reflectors with different angles of incidence. By way of example, at a particular time of day, the backside of a reflector directed in a generally easterly direction may be substantially normal to incident solar radiation while the backside of a generally westerly directed reflector may be angled by 45.degree., 60.degree. or even a greater angle of incidence to the incident solar radiation. Consequently, little solar radiation impinges on the westerly reflector.
The amount of differential angulation of the reflectors to the incident solar radiation depends on the specific design of the antennas, particularly with respect to the placement of feed elements on the body of the satellite relative to the front surfaces of the reflectors. As a result of the differential interception of the solar radiation by the two antenna reflectors, the total solar induced force on a reflector, as computed by integration of the solar pressure across the back surface of the reflector, is substantially greater for the reflector which is normal to the incident solar rays than for the solar induced force upon the reflector which is angled away from the incident solar radiation. Each of these forces introduces a torque about the body of the satellite. Since the forces differ in magnitude, the resulting torques differ in magnitude, this resulting in a net torque which tends to rotate the satellite away from the desired orientation away from the earth. The direction of the torque may change with time of day as the relative position of sun to satellite changes.